Lifting devices of the class referred to as crane hoists usually consist of a block and hook assembly suspended from a frame member that in turn is supported by one or more rollers on the lower flange of an I-beam. The block assembly is typically suspended from the frame by a cable that passes through a pulley sheave on the block. The cable is used to raise and lower the block assembly to thereby raise and lower a load supported by the hook. The hoist is either rolled by hand along the lower flange of the beam or a single roller is power driven to move the hoist along the beam.
These crane hoists have several drawbacks. First, having the block and hook assembly suspended by a cable or chain passing through a single pulley sheave is inherently unstable. A single pulley arrangement allows the suspended load to freely swing on the chain upon the slightest change in the speed or direction of movement of the hoist on the beam. In addition, the load can easily rotate, causing the chain to twist. This can result in damage to the chain and the load and possible injury to the operator or nearby workers. Furthermore, it has been found that a single drive wheel will lose traction and slip as it encounters irregularities along the lower flange of the I-beam. The resultant erratic stopping and starting of the crane hoist can cause swinging of the suspended load, resulting in additional forces being placed on the crane hoist and the beam. Having a drive wheel mounted on each side of the I-beam has been considered impractical because of the clearance required below the beam for drive shafts, which would interfere with the raising of the block, and, thus, limit the height to which a load can be raised.
Another drawback to these crane hoists is the uneven distribution of loads on the load supporting chains as a result of pulley sheave inefficiency. This uneven distribution of loads can cause the block to diverge from a level position as it is being raised and lowered. These and other disadvantages are overcome in the present invention.